The invention relates to a method for depth compensation of amplitudes of echo-signals in an ultrasonic measuring device which uses the pulse-echo-technique, in the course of which as a first step ultrasonic pulses are sent from the probe of said ultrasonic measuring device into a trial object containing several, as far as possible identical, flaws in known distances from the surface and lined up in depth reflecting the pulses, such that they return into the same or another probe of the ultrasonic measuring device, where they arrive in a temporal order corresponding to the spatial order in depth and give rise to electrical signals with different amplitudes at the output of the receiver unit of the ultrasonic measuring device, the electrical signals being available as so-called nodes of interpolation with their amplitude and time values, and a digital correction signal being obtained from the deviation of the signal amplitudes at every single node of interpolation from a corresponding reference amplitude, the correction signal being stored and used, after conversion to an analogue correction signal, to control an adjustable amplifier, so that in a subsequent measurement of the same material flaws of the same kind will yield equally high amplitudes of echo signals irrespective of depth.
In high-quality ultrasonic devices, particularly in ultrasonic devices intended for nondestructive testing of materials, a depth compensation function is provided to compensate for the dependence of echo amplitudes on depth. In the method previously known from DE 26 23 522 C2, the material to be tested is divided into digital depth intervals, and for every digital depth interval a corresponding digital signal correction value is calculated. These signal correction values are stored. During the measurement proper the signal correction values are converted into analogue values which are used to correct the values of the ultrasonic signal received. A method with analogue storage of the correction values is known from U.S. Pat. No. 3,033,029.
The method described in DE 26 23 522 C2 which is representative for its class is disadvantageous in its necessity to store one amplitude value for each depth interval, and hence for each time increment. This leads to high storage requirements. The required storage size is calculated as the ratio of the depth range and the measure in depth of one depth interval. The potential penetration depth range is predetermined by the storage size. Finally, it is disadvantageous that the nodes of interpolation must be fitted into the given grid of depth intervals, and preferably nodes of interpolation must be equidistant.